What is the difference between...?

Alone

According to the Oxford dictionary, the definition of alone is, "having no one else present."For example, 'she lives alone' or 'he was eating at the restaurant alone.'

Solitude

According to the Cambridge dictionary, the definition of solitude is, "the situation of being alone, often by choice."For example, 'she prefers the solitude of the country to the crowds of the city.' Or 'he prefers a time of solitude at home after his in-laws leave.' Or 'she entered a space of solitude in her art studio to work.'

Loneliness

The definition of loneliness is the feeling one has when what one desires in terms of relationship/connection does not currently exist. Thus, a gap exists between what one desires and what one currently experiences having. For example, 'she feels lonely since the death of her grandfather because she misses their phone conversations and lunches together.' Or 'she feels lonely at home with her husband because the intimacy she desires is not present between them.' Or 'he feels lonely since moving to Paris because he wishes he had a group of friends to hang out with.'

One key difference is that unfulfilled desire is at the heart of the human experience of loneliness. Whereas, unfulfilled desire is not at the heart of the definition of alone or solitude. Another key difference is choice. Being alone, or in solitude, are often the result of a deliberate choice.

For example, 'he chose to go for a walk alone tonight' or 'she planned a weekend of solitude at her seaside home for a time of meditation and reflection.' In contrast, people generally do not choose to feel lonely.

So let us be clear that when we speak of loneliness, we speak of loneliness.